The invention relates to a method of manufacturing an optical component, in which method a substrate is successively provided with an orientation layer and an optically anisotropic layer of a material containing monomers which are provided with at least one covalently bonded reactive group, whereafter said optically anisotropic layer is polymerized. The method also relates to a polarization-sensitive beam splitter (PBS) and to a cholesteric filter, which are manufactured by means of this method.
A method of the type mentioned in the opening paragraph is known per se. For example, in European Patent Application EP 576 072-A1, a description is given of this method of manufacturing of a polarization-sensitive beam splitter which comprises at least a wedge-shaped element of a birefringent material, the so-called wedge-shaped PBS. The material used consists of a uniaxially oriented polymer which is formed from a polymerized, liquid-crystalline monomer composition. At least a number of the monomers of the monomer composition are provided with at least one covalently bonded reactive group. Said composition is cured by means of polymerisation between two substrates which are arranged in such a manner relative to each other that they form a wedge, the facing surfaces of said substrates being provided with an orientation layer.
Such a method is also suggested in the non-prepublished European Patent Application 95201770.5 (WO 97/01779). In said Patent Application, a description is given of the manufacture of a different type of polarization-sensitive beam splitter. This beam splitter is flat and comprises a substrate with an orientation layer on which a structured, optically anisotropic layer is provided. This layer includes areas with birefringent material and areas with isotropic material. This material is provided in two steps by means of a replica process. In this process, use is made of a pressing mould having a structured pressing surface, for example, in the form of a zigzag structure or a saw-tooth structure. First, the optically anisotropic material is provided on the orientation layer in accordance with a specific structure by means of the replica process and then polymerized. After the pressing mould has been removed, the resultant grooves are filled by means of the isotropic material (for example by spinning), whereafter also this material is polymerized. By means of this method, a so-called flat, polarization-sensitive beam splitter (flat PBS) is obtained.
Also in European Patent Application EP 643121 A1, a description is given of such a method, which is used to manufacture a switchable cholesteric filter. This filter comprises an optically active layer with liquid-crystalline material, which is sandwiched between two substrates which are each provided with an orientation layer. Said liquid-crystalline material comprises the cholesterically ordered polymerization product of a mixture of monomers of which a part contains at least one covalently bonded reactive group. By virtue of the presence of a small quantity of monomers with two covalently bonded groups, a slightly cross-linked network is formed by polymerization.
The known method has drawbacks. One of the drawbacks is that the adhesion between the polymerized, optically anisotropic layer and the substrate carrying the orientation layer is insufficient. In the manufacture of the wedge-shaped polarization-sensitive beam splitter ("wedge-shaped PBS") this disadvantage manifests itself when one of the two substrates between which the wedge-shaped optically anisotropic material is polymerized is removed. Visual inspection has revealed that when one substrate is detached, the adhesion between the wedge-shaped element and the other substrate is seriously affected. Sometimes said operation even leads to the formation of cracks in the wedge-shaped element. These effects have a negative influence on the optical quality of the wedge-shaped PBS. This is considered to be a serious disadvantage.
In the manufacture of the other type of polarization-sensitive beam splitter ("flat PBS"), a similar problem occurs. This problem manifests itself during the removal of the pressing mould, after the optically anisotropic layer has been provided and polymerized. Sometimes parts of the applied layer adhere so firmly to the surface of the mould that they are pulled loose from the substrate. Consequently, also in this case the adhesion between the optically anisotropic layer and the substrate provided with the orientation layer is insufficient.
Also in the case of the cholesteric filters manufactured by means of the known method, the adhesion between the polymerized optically anisotropic material and the substrate gives rise to problems. For example, some time after the manufacture of the filters, undesirable structures (cauliflower effect) form on the filters. Close examination of these filters has revealed that the cholesteric material becomes detached from the substrate. This can possibly be attributed to tensile stresses which develop during the polymerization of the optically active layer and the associated shrinkage.